The present invention relates generally to optical scanning or detecting devices and, more particularly, to a scanning device capable of step stare scanning to thereby enhance the field of view of a stare scanning array.
Optical scanning devices have long been used in tracking and sensing systems such as in aircraft forward looking infra-red radar (FLIR) systems. In a typical optomechanical scanner, a rotating scanning mirror is generally used to create a linear scanning pattern over a target. Coupled with the forward motion of an aircraft, for example, the linear scanning pattern generated by the rotating scanning mirror is converted into a saw-tooth or triangular scanning pattern and an area can be effectively scanned. Scanning is further accomplished in passive and active modes. In a passive mode, the scanning device simply "looks" at a target area and receives signals emanating from the scan target and reflects them to a detector, such as in a system set up to scan infra-red radiation emanating from the target. In an active scanning mode, the scanning device also includes a signal source, such as a laser, which sends out a signal. The scanning device receives a portion of the signal which is reflected back from the target at the detector. Information derived from the signal reflected back to the scanner allows for the determination of a number of pieces of information such as the distance of the object from the scanner or the general shape of the object. Often the scanning application requires an area to be scanned very quickly. While there is generally no problem with having the mechanical elements of the scanner operate at higher frequencies, scanning devices are often limited by the rate at which the detector elements can receive information.
Stare scanning devices, i.e., devices having multiple detector elements assembled into an array, allow for greater reception of information and thus increase the speed of the scanning system. In such devices, the array of elements simultaneously receive information from an area within its field of view in what is known as electronic scanning- These devices are also capable of operating in passive or active scanning modes. The disadvantage of stare scanning arrays is that they have a limited field of view generally defined by the number of detectors in the array. An illustrative example of this narrow field of view is the relatively limited image viewed through the lens of a camera versus the image viewed by the human eye. Thus there is a need for enhancing the field of view of stare scanning arrays.
One method of enhancing the field of view of stare scanning devices is to simply construct larger scanning arrays. Unfortunately, the large number of detector elements required to significantly enlarge the field of view increase the size, weight, cost and technical complexity of the scanning device. A second method is to create a mechanical apparatus which moves or scans the stare scanning array at different view angles. The apparatus moves the array to a viewing angle, allows the scanner to scan at that viewing angle for a period of time, moves the array to new viewing angle and so on. The start-stop motion created by moving the scanner to different viewing angles, however, creates mechanism accelerations, requires large amounts of power, and imparts forces on the mounting system thus requiring sturdier, and necessarily heavier mounting structures. All of these characteristics are undesirable particularly if the scanning apparatus is to be used in aircraft or spacecraft applications.
Another method of enhancing the field of view of stare scanning arrays is to create a scanning device that will scan at a number of different viewing angles and reflect the signals received from a particular viewing angle to a single stare scanning array. Such an arrangement advantageously combines the increased field of view available with optomechanical scanning devices with the speed of electronic scanning with a stare scanning array. Prior devices have used oscillating mirrors or vertically and horizontally rotating prisms to hold or "stare" while scanning at a particular viewing angle and then move or "step" to a new viewing angle, scanning at this new viewing angle and so on. While these systems are adequate for providing the appropriate step stare scanning pattern, they also suffer the disadvantages associated with causing the scanning apparatus to make start-stop motions necessary to adjust the mirror or prism from one viewing angle to another viewing angle. Therefore, it would be desirable to create the step stare scanning pattern with a constant motion.